1. Field of the Invention
The invention relates to fabrication of optical fiber, including fabrication of preforms from which optical fiber is drawn.
2. Discussion of the Related Art
Optical fiber is produced from a glass preform. The preform is generally arranged vertically in a draw tower such that a portion of the preform is lowered into a furnace region. The portion of the preform placed into the furnace region begins to soften, and the lower end of the preform forms what is known as the neck-down region, where glass flows from the original cross-sectional area of the preform to the desired cross-sectional area of the fiber. From the lower tip of this neck-down region, the optical fiber is drawn.
The optical fiber typically contains a high-purity silica glass core optionally doped with a refractive index-raising element such as germanium, an inner cladding of high-purity silica glass optionally doped with a refractive index-lowering element such as fluorine, and an outer cladding of undoped silica glass. In some manufacturing processes, the preforms for making such fiber are fabricated by forming an overcladding tube for the outer cladding, and separately forming a core rod containing the core material and inner cladding material. Overcladding tubes are capable of being formed by a sol-gel process, as discussed, for example, in co-assigned U.S. Pat. No. 5,240,488, or by drawing the tubes from a silica billetxe2x80x94such tubes are available commercially. The core rods are fabricated by any of a variety of vapor deposition methods known to those skilled in the art, including vapor axial deposition (VAD), outside vapor deposition (OVD), and modified chemical vapor deposition (MCVD). MCVD, for example, involves passing a high-purity gas, e.g., a mixture of gases containing silicon and germanium, through the interior of a silica tube (known as the substrate tube) while heating the outside of the tube with a traversing oxy-hydrogen torch. In the heated area of the tube, a gas phase reaction occurs that deposits particles on the tube wall. This deposit, which forms ahead of the torch, is sintered as the torch passes over it. The process is repeated in successive passes until the requisite quantity of silica and/or germanium-doped silica is deposited. Once deposition is complete, the body is heated to collapse the substrate tube and obtain a consolidated rod in which the substrate tube constitutes the outer portion of the inner cladding material. To obtain a finished preform, the overcladding tube is typically placed over the core rod, and the components are heated and collapsed into a solid, consolidated preform, as discussed in co-assigned U.S. Pat. No. 4,775,401.
Optical fiber manufacture has reached a very sophisticated level of development. Yet, in some cases, fiber specifications are so stringent that it is difficult to develop processes capable of meeting such specifications. For example, the properties of many high-end fibers, particularly the dispersion properties, are extremely sensitive to variations in fiber core diameter. In fact, calculations for some commercially-available fiber have shown that as little as a xc2x11% variation in core diameter induces up to a xc2x114% variation in dispersion. Due to this dispersion effect, specifications for such fiber generally allow less than xc2x12% variation in core diameter. With these stringent requirements, it is sometimes difficult to achieve adequate yields in manufacture.
In addition to problems with core diameters, there are numerous fiber designs based on particular core diameter configurations, where the designs are intended, for example, to provide specific dispersion properties. Yet, there are no existing processes that allow production of such fibers in a feasible, commercially-acceptable manner. The designs remain, therefore, primarily theoretical.
Thus, it would be desirable to have a process capable of providing a core rod having substantially uniform core diameter, and, advantageously, also capable of tuning the core diameter profile to provide particular fiber properties.
The invention relates to a process capable of not only providing a substantially uniform core diameter, but also of tuning the core diameter profile for a particular fiber design. According to the invention, a core rod, typically silica-based, is traversed by a heat source along the rod""s longitudinal axis, to provide heated, softened regions. During the traverse, compressive or tensile movements are provided along the rod""s longitudinal axis, these movements inducing, respectively, increases or decreases in the core diameter at the softened regions.
In particular, as the heat source traverses the core rod, the source heats discrete regions of the rod above the rod material""s softening point. (Softening point indicates the conditions at which the material reaches a viscosity at which it is possible to induce flow, e.g., for silica the softening point generally occurs when the material reaches a viscosity of about 107.6 poise.) If the core diameter at the particular region being heated is desirably larger, a compressive movement is applied to expand the diameter of the softened region by viscous flow (the movement expands the diameters of both the core and the overall rod). Alternatively, if the core diameter is desirably smaller, a tensile movement is applied to stretch that softened region and thereby reduce the core diameter (and overall rod diameter), again by viscous flow. These compressive and/or tensile movements are continued, and varied in degree to provide the appropriate diameter expansion or contraction, as the heat source traverses the core rod. As the heat source moves past the region, the diameter adjustments are essentially locked in place upon cooling. It is then possible to form a fiber preform containing the resultant rod by conventional techniques, and to draw fiber therefrom.
By providing selective core diameter increases and/or decreases across the entire length of the core rod, a desired core diameter profile is attained. For example, it is possible to attain a substantially uniform core diameter, e.g., where the core diameter over at least 90%, optionally 100%, of the rod length is within 0.2% of the average core diameter, optionally within 0.1%. It is also possible to provide varying core diameter profiles to provide particular properties, such as systematically varying dispersion. For example, future systems may employ fiber having pre-selected dispersion variations over a particular transmission length, in order to reduce non-linearities. In addition, the ability to increase core diameter and core rod diameter in a controlled manner makes fabrication of larger core rods possible, which in turn makes larger fiber preforms possible, e.g., preforms capable of providing at least 1200 km, or even 2400 km, of 125 xcexcm diameter fiber.